Although, by definition, cancer cells have a deregulated cell cycle, tumors often retain an intact G1 cell cycle checkpoint response via p53-dependent transactivation of p21, which then inhibits cyclin-dependent kinases (Cdk) and prevents G1/S transition. More importantly, tumors retaining this checkpoint response are also sensitive to therapeutic agents. Indeed, a strong positive correlation exists between the ability of tumor cells to arrest in G1 and sensitivity to platinum-based agents (cisplatin, tetraplatin and oxaliplatin) in the NCI 60- cell line tumor panel. This is also consistent with the finding that mutant-p53 tumor cells transfected with a p21 expression vector are sensitized to cisplatin. Moreover, our preliminary data demonstrate that p21- knockout induces resistance to cisplatin and to a novel analog DAP, which was designed to circumvent cisplatin resistance. The analog DAP is highly interesting among DNA-damaging agents in that it only inhibits G1-phase Cdk to selectively induce G1 arrest, and this consolidates the relationship between G1-phase Cdk inhibition by p21 and cell death. However, the underlying basis for this relationship has not been clearly defined in the literature, possibly due to signaling interference from the cross talk caused by inhibition of Cdk in S- and G2-phases, which are linked to cell-survival signaling. The availability of DAP, which does not inhibit the Cdk in these two phases, provides an important and timely opportunity to delineate this relationship so that rational approaches could emerge to improve the spectrum of therapeutic response. In preliminary studies, we have made a seminal observation that G1-phase Cdk complexes inhibited by DAP-induced p21 are substantially larger in size than the corresponding active complexes in control cells. Proteomic analysis of the inhibited complex identified the expected p21, but revealed PCNA as a novel recruit, which potentially links inhibition of G1-phase Cdk activity to platinum-mediated cell death. Therefore, we hypothesize that tumor cell death by platinum-based agents is dependent on p21 function at the G1-phase Cdk locus and this is facilitated by PCNA recruitment. We will address this hypothesis with three specific aims, which will better define our understanding of the process of G1-phase inhibition, and link this p21-dependent event to cell death. Moreover, comparative studies with platinum complexes will allow us to appreciate a mechanism of cisplatin resistance that is circumventable by non-cross-resistant analogs.